A metallic staple acts as a needle to fasten a plurality of papers together or a plurality of structural members together, and is largely classified into stationery staples used in general households, offices, and the like; industrial staples used for houses, furniture, and the like; and sealing staples used for various corrugated cardboard boxes and the like. A device which punches out staples one by one from a strip of a predetermined number of metallic materials (staples) adhered to each other in order to bond targets together, is called a stapler. Furthermore, pin nails or finish nails which are metallic nails, are used as nails to fix a decorative board such as an acoustic board as a ceiling finishing material to the ceiling. The device using such pin nails or finish nails are called a tacker, and there are those which have manual-type, electric-type, and air-type driving sources. In the present application, these pin nails and finish nails as well as the aforementioned staples are collectively called fasteners. While the description below is of the conventional art of staples for the sake of convenience, the description also applies to the conventional art of pin nails and finish nails.
The coating agent for metallic staples needs to have a holding force to hold a predetermined number of staples in a bonded state until they are housed in a stapler, as well as flexibility so that each staple does not easily separate off during handling. On the other hand, when fastening targets with a stapler, it is necessary that only one staple to be punched out is easily separated from a strip of staples. Furthermore, the coating agent needs to be excellent in blocking resistance so that during production or market distribution of staples on which the coating agent is applied, the surfaces of the strips of staples do not stick on to each other when they contact with each other.
In order to satisfy the aforementioned conflicting requirements, a solvent-type rubber-base adhesive is mainly used currently as a coating agent. As a typical solvent-type rubber-base adhesive used for metallic staples, commonly used are those in which nitrile rubber and vinyl chloride or vinyl chloride-vinyl acetate copolymerized resin are formulated in addition to an organic solvent as a solvent. However, there is no conventional solvent-type rubber-base adhesive which completely satisfies the aforementioned required performance, and, in particular, when the performance as a staple is emphasized, in order to improve blocking resistance, another lacquer-base coating agent making a hard coating film needs to be coated on top of an ordinary coating agent, or the amount of coating needs to be strictly controlled, which has caused the problem of low productivity.
Another large problem in conventional solvent-type rubber-base adhesives is that vinyl chloride (vinyl chloride resin), one of their components reportedly has a possibility of generating dioxins which are harmful to humans or environment when burnt at a low temperature (850° C. or less). The same concerns remain even with commonly-used copolymerized products of vinyl chloride and vinyl acetate, i.e., vinyl chloride-vinyl acetate copolymerized resins.
Under such circumstances, provision of a coating agent for metallic staples which requires no halogen-containing compound such as vinyl chloride resin without compromising the performance is now being demanded. In response to such required performance, as a coating agent containing no vinyl chloride resin, there is known, for example, a urethane-base hot-melt adhesive (Patent Document 1) and an acrylic resin base adhesive (Patent Document 2). No coating agent, however, has been provided yet which has the flexibility of bonding metallic materials together, while not letting them easily fracture during handling, as well as a proper degree of shearing force which enables separation one by one of such bonded metallic materials by a stapler, both of which are required of staples.    [Patent Document 1] Japanese Laid-open Patent [Kokai] Publication No. Hei 10-29384 (1998)    [Patent Document 2] Japanese Laid-open Patent [Kokai] Publication No. 2003-90313